There is a continuous effort to develop sensors that are lower in cost and smaller in size, yet are characterized by high reliability, sensitivity and linearity. Pressure sensors finding wide acceptance on the basis of furthering these characteristics include those that utilize semiconductor materials with a micromachined sensing diaphragm, a notable example being micromachined single-crystal silicon pressure transducer cells manufactured using semiconductor fabrication processes. In the processing of such cells, a thin diaphragm is typically formed in a silicon wafer through preferential chemical etching. Ion implantation and diffusion techniques are then used to drive doping elements into the diaphragm, forming piezoresistive elements whose electrical conductivity changes with strain such that deflection of the diaphragm causes a change in resistance value of the piezoresistive elements, which can then be correlated to the magnitude of the pressure applied to the diaphragm. Silicon pressure sensing cells that rely on piezoelectric and capacitive sensing have also been produced.
Diaphragms of single-crystal silicon pressure sensing cells are typically small, rarely exceeding a few millimeters in width, and are very thin, with a thickness of often less than 100 micrometers. The use of standard single-crystal silicon wafers and standard semiconductor device fabrication processes allows many such cells to be fabricated from a single wafer, providing some economy of scale. However, silicon is susceptible to chemical attack and erosion by various media, particularly in applications where a high-pressure medium is to be sensed, e.g., automotive applications that involve sensing brake fluid, oil, transmission fluid, hydraulic fluid, fuel and steering fluid pressures. Consequently, for high-pressure sensing applications, a silicon pressure sensing cell requires some form of protection, such as a protective enclosure that complicates the manufacture and increases production costs.
Silicon sensors have found limited use for sensing force and displacement because of the brittle nature of the silicon materials. As a result, current methods for producing force and displacement sensors include the use of metal foil strain gauges placed over a diaphragm, electrostatic or capacitive sensing structures, magnetic or ultrasonic measurement techniques, and the compression of conductive gels. Attempts to employ silicon micromachined sensing cells to sense force and displacement have included enclosing a silicon cell in a fluid or gas-filled container. However, compression of the container does little to change the pressure of the fluid, with the result that the sensor output does not change significantly with force. U.S. Pat. No. 5,353,003 to Maurer discloses a force sensor that employs a shaft and resilient material to transmit force to a silicon dioxide force-sensing diaphragm. However, as a moving component that must remain free to move throughout the useful life of the force sensor, the requirement for a shaft to transmit force to the silicon dioxide diaphragm limits the usefulness and life of this type of sensor.
In view of the above, it is apparent that the above sensors and/or their manufacturing processes have limitations and drawbacks. For example, silicon sensing cells adapted for high pressure applications suffer from low sensitivity and complicated manufacturing processes that render the sensors incompatible with mass-production applications, while silicon sensing cells adapted for sensing force have found limited use due to the brittle nature of silicon. Accordingly, there is a need for a sensor that attains the high reliability and sensitivity associated with silicon sensing cells, yet is capable of sensing force and displacement for a variety of applications, is relatively uncomplicated and is low in cost.